Magnetoresistive (MR) materials experience changes in electrical resistivity when exposed to external magnetic fields. Such materials have a wide range of use because of their ability to detect and differentiate magnetic field strength. One of the more common uses of this technology is in magnetic data storage where data is stored on a magnetic media by varying the magnetic fields of small magnetic particles in the media. The media""s magnetic field is made to fluctuate by a write head in proportion with the information to be stored on the media. The fluctuations contained in the media can subsequently be retrieved using a read head.
Standard magnetoresistive sensors, which may be used in read heads to detect the magnetic fields on the magnetic data storage media, use a detection element constructed of a magnetic material subjected to an electrical current. When placed in the presence of an external magnetic field, such as that generated by magnetic storage media, the sensor is able to measure the existence and strength of the external magnetic field through correlation with measurements of the resistivity experienced by the electrical current. The sensor becomes more or less resistive depending upon the magnetic field of the media. This allows, for example, information on magnetic media to be read by measurement of the current flow through the sensor.
Certain magnetoresistive sensors exhibit an increased sensitivity to external magnetic fields. Such sensors experience relatively larger changes in resistivity compared to normal magnetoresistive sensors. These sensors exhibit what is known as the giant magnetoresistive (GMR) effect. Magnetic multilayers, granular solids, and other materials with heterogeneous magnetic nanostructures exhibit GMR effects. Specifically, these structures exhibit a negative GMR effect in which the magnetoresistance decreases with an increase in the magnitude of an external magnetic field. Prototype GMR structures such as multilayers and granular solids require magnetic fields on the order of 10 kOe to fully realize the GMR effect.
The effectiveness of a giant magnetoresistive construction is often measured in terms of its maximum MR effect size denoted by a ratio or percentage figure dependent upon the change in electrical resistance of a material when exposed to an external magnetic field. Currently, most read heads in the magnetic recording industry utilize the anisotropic MR effect in permalloy which has an MR effect of about 2%, or a ratio of 0.02. Recently, most sophisticated read head made of spin-valve GMR structures have been commercialized with an effect size of about 5-10%, or a ratio of 0.05 to 0.10. Maximum MR effect size is dependent upon the resistance of the material at zero magnetic field and the resistance of the material at magnetic saturation. The strength of the saturation magnetic field (HS) is determined by the composition of the material and is the field at which the largest MR effect is realized. The largest MR effect values ever reported have been 150% at low temperatures (e.g., 4 K) and 80% at room temperature at a saturation field of about 20 kOe. Most reported MR values, and particularly those in devices, are much smaller, i.e., in the range of 5% to 10% at room temperature.
Important characteristics for MR devices include the detection limit (i.e., the smallest magnetic field that can be detected), sensitivity (i.e., the percent change of MR per unit magnetic field), and the dynamic range (i.e., the range of magnetic field that can be detected). Not all MR devices value these characteristics in the same way. For example, in read head applications, the detection limit and sensitivity are important, whereas in current sensing applications, the detection limit, sensitivity, and dynamic range are all important. In general, a large MR effect size is always advantageous since it directly improves the detection limit and the sensitivity. In addition, a simple magnetic field dependence (e.g., non-saturable) of the MR and a large dynamic range are desired for field sensors.
Bismuth (Bi) is a semi-metallic element with unusual transport properties, including a large MR and Hall effect. The electronic properties of Bi, which are very different from those of common metals, are due to its highly anisotropic Fermi surface, low carrier concentration, small carrier effective masses, and long carrier mean free path. As a result, bulk single crystals of Bi are known to exhibit a very large MR effect.
Unfortunately, the fabrication of high quality Bi thin films, a necessary requirement for most device applications, is known in the art to be difficult. Deposition of MR thin films generally occurs through one or a combination of the following techniques: chemical vapor deposition (CVD), physical vapor deposition (PVD) (e.g., sputtering, evaporation, etc.), or electrochemical deposition. Bi thin films made by traditional vapor deposition such as sputtering and laser ablation are of very poor quality and exhibit a polycrystalline structure with small grains. As a result, those Bi films exhibit very small MR, on the order of 1-10% at 300 K under a field of 1 Telsa (T), which is unsuitable for applications. Previously, only Bi films made by molecular beam epitaxy, which is a prohibitively expensive method, yielded high quality Bi thin films with large MR.
Electrochemical deposition offers precise control over the microstructure and a process which can be performed economically and reliably. This translates into the possibility for mass production of high quality materials.
However, electrochemical deposition processes used to deposit bismuth directly onto a substrate have thus far been insufficient to produce MR effect levels above 150%. Processes involving the direct electrochemical deposition of bismuth onto substrates or a metallic underlayer have generally resulted in polycrystahine films with voids and other defects. One such process is described in U.S. Pat. No. 5,256,260 (Norton et al.). This process utilizes a constant-current molten salt electrocrystalization bath in which bismuth ions are complexed with a barium-based component and a bismuth-based component. Current electrochemical deposition techniques for bismuth onto a substrate result in polycrystalline films which do not allow for realization of very large MR effects.
The invention is directed to the use of electrochemical deposition to fabricate thin films of a material (e.g., bismuth) exhibiting a superior magnetoresistive effect. The process in accordance with a preferred embodiment produces a thin film of bismuth with reduced polycrystallinization and allows for the production of single crystalline thin films. Fabrication of a bismuth thin film in accordance with a preferred embodiment of the invention includes deposition of a bismuth layer onto a substrate using electrochemical deposition under relatively constant current density. Preferably, the resulting product is subsequently exposed to an annealing stage for the formation of a single crystal bismuth thin film. The inclusion of these two stages in the process produces a thin film exhibiting superior MR with a simple field dependence suitable for a variety of field sensing applications.